The present invention relates to a digital radio communication system. More specifically, the present invention relates to a relay apparatus in a digital radio communication system and a relay method thereof.
As shown in FIG. 6, a currently practically-used digital radio communication system is a system configured to provide a communication connection service between a control station 51 and a plurality of mobile stations M1, M2, . . . in a communication area 52 (also called a communication zone) of the control station 51 or a communication connection service between a plurality of mobile stations m1, m2, . . . in a communication area 54 of a relay station 53 and the control station 51 or the plurality of mobile stations M1, M2, . . . . A base station 55 may be located near the control station 51 or may be located in a place away therefrom. In this case, the control station 51 and the base station 55 are typically connected by a cable or a microwave line. FIG. 6 shows the case that the control station 51 and the base station 55 are located in the same place. The control station 51 connects a communication between the base station, the relay station and the plurality of mobile stations in the digital radio communication system and maintains and manages a service area. Line control equipment is provided in the control station 51 to control a call from the mobile station or a communication route setting. In FIG. 6, the relay station 53 is located in the communication area 52 of the control station 51, but is not necessarily located in the communication area 52. The control station and the relay station are connected by microwave multiplex radio transmission or a digital dedicated line.
FIG. 7 shows radio carrier frequency allocation which is allowed to be used in a narrow band digital system including a regional mobile telecommunication system using a digital radio technique in Japan. In FIG. 7, in the upward direction, that is, in the direction of mobile station→relay station→control station, based on 262 MHz, a 4 MHz band is allowed to have 160 waves (f1, f2, . . . ) with a 25 KHz width. In the downward direction, that is, in the direction of control station→relay station→mobile station, based on 271 MHz away from 262 MHz in the upward direction by 9 MHz, a 4 MHz band is allowed to have 160 waves (F1, F2, . . . ) with a 25 KHz width. In a communication of the digital radio communication system, the frequencies of f1, f2, . . . in the upward direction and F1, F2, . . . in the downward direction are used. Each system can use one or a plurality of radio carriers as a control carrier and the remainder as a communication carrier corresponding to its size. Needless to say, such frequency allocation is different by region and country. The standard of the digital radio system in Japan is defined by ARIB (Association of Radio Industries and Businesses) Standard-T79 (issued by Association of Radio Industries and Businesses in September, 2001) (hereinafter referred to as ARIB STD).
When such digital radio communication system performs a radio communication between the mobile stations in different zones such as a control station zone and a relay station zone, as in an analog radio communication system, the mobile station in the relay station zone, e.g., the mobile station m1 performs a radio communication with the mobile station in the zone of the control station 51, e.g., the mobile station M1 via the relay station 53. A radio communication channel controlled by the line control equipment of the control station is used in this radio communication.
FIG. 4 shows a specific configuration of an example of a digital radio communication system using a prior art radio relay system. FIG. 4 is a diagram of assistance in explaining an operation when the mobile station M1 calls the mobile station m1 in FIG. 6. The operation will be described in the downward direction, that is, in the direction of control station→relay station→mobile station. When there is a calling (connecting requirement) from the mobile station M1, the control station 51 (or via the base station 55) must detect the calling and establish a communication route from the control station 51 via the relay station 53 to the mobile station m1. When the control station detects the calling from the mobile station M1, the control station connects line control equipment 401 and a line I/F (interface) 404 of a relay station 400 via a transmission path 415 such as a digital dedicated line or microwave multiplex radio transmission.
A signal TS propagated via the line 415 is a digital signal of a frame structure and is indicated by a transmission signal TS of FIG. 8. In FIG. 8, one frame has a length of 40 msec so that the transmission signal is composed by repeating the frame. One frame also has 8 channels (CH1, CH2, . . . CH8) and one channel length is 5 msec. The CH1 is used as a control channel, for example and the CH2 to CH8 are used as a traffic channel, for example.
A signal received by the line I/F 404 using the control channel is applied to a data substituting unit 406 (hereinafter called a data converter 406). The data converter 406 performs a radio connection with the mobile station located in the relay station zone so as to convert a transmission format. That is, the signal is operated in synchronization with the line control equipment 401 at timing obtained from a timing generator 405 and is then converted to a control channel signal of a radio section based on control of a control unit 407. As shown in FIG. 8, the TS transmission signal having 8 channels in one frame is converted to two transmission signals C1 and C2, each of which has 4 channels in one frame. One frame of the transmission signals C1 and C2 has the same length of 40 msec as that of one frame of the transmission signal TS. The channel length of the transmission signals C1 and C2 is 10 msec.
The control channel format-converted by the data converter 406 is added a preamble, a synchronous word, a control signal and an error correction code conforming to the standard by a coder/decoder circuit 414 (hereinafter called a channel codec) for coding. A signal of the coded control channel is applied to a transmission modulation unit 411-1 and is then converted to a signal permitting a digital radio communication with the mobile station. The control channel which is a radio channel having a slot for common use is defined as a control channel in the ARIB STD.
A signal received by the line I/F 404 using the traffic channel is applied to the data converter 406, as in the control channel. A signal of the traffic channel applied to the data converter 406 is operated in synchronization with the line control equipment 401 at timing obtained from the timing generator 405, extracting only voice data from the transmission signal TS, as shown in FIG. 8, based on control of the control unit 407. The CH2 to CH4 are converted to the transmission signal C1 and the CH5 to CH8 are converted to the transmission signal C2 so as to be supplied to the channel codec 414. The channel codec 414 adds a preamble, a synchronous word, and an error correction code to the traffic channels of the transmission signals C1 and C2 for coding, which are then baseband signals of the radio section. The transmission signals C1 and C2 are applied to the transmission modulation units 411-1 and 411-2, respectively. The transmission signals C1 and C2 applied to the transmission modulation units 411-1 and 411-2 are converted to signals permitting a digital radio communication with the mobile station and are then amplified by power amplifier units 410-1 and 410-2 to be supplied to an antenna 408 via a transmission filter 409. The transmission signals C1 and C2 are outputted from the antenna 408 at the frequencies F1 and F2 as a communication wave in the downward direction, as described above. The antenna 408 of the relay station and an antenna 402 of the mobile station are connected by a digital radio channel. A transmit-receive unit 403 of the mobile station can receive the control channel signal and the traffic channel signal from the relay station. The traffic channel which is a radio channel having an individual allocation slot is defined as a traffic channel in the ARIB STD.
There are various digital modulation operations for use in the transmission modulation units 411-1 and 411-2. A predetermined modulation operation suitable for the digital radio communication system is used. A π/4 shift QPSK modulation operation is often used.
The operation in the upward direction, that is, mobile station→relay station→control station will be described. The signals at the frequencies of f1 and f2 transmitted from the transmit-receive unit 403 via the antenna 402 are received as reception information by the antenna 408 of the relay station and are then subjected to separation of frequency by a reception filter 413 for band limit to be outputted to reception demodulation units 415-1 and 415-2. The reception demodulation units 415-1 and 415-2 demodulate the reception information by a predetermined demodulation operation for output to the channel codec 414. The channel codec 414 performs decoding including error correction conforming to the standard. A signal process reversed from that in the downward direction is performed to provide the transmission signals C1 and C2 of FIG. 8, thereby obtaining the control channel signal and the traffic channel signal. The control channel signal and the traffic channel signal are applied to the data converter 406 and are then subjected to channel conversion reversed from that in the downward direction, thereby obtaining the transmission signal TS of FIG. 8. The control unit 407 selects necessary data and then transmits the selected signal from the line I/F 404 via the transmission path 415 to the line control equipment 401 to establish the communication route.
The above-described prior art digital radio relay system has the following problems.
(1) When the line control equipment of the control station and the relay station are connected by a digital dedicated line, a digital line of other companies is borrowed. Its use fee is required. The cost is high for a privately-owned radio communication system.
(2) When the line control equipment of the control station and the relay station are connected by a digital dedicated line, the digital dedicated line may be disconnected due to an external factor such as a disaster. The system cannot be used in the digital radio communication system for disaster prevention, which is a serious disadvantage in reliability.
(3) When the line control equipment of the control station and the relay station are connected by microwave multiplex radio transmission, the cost of the microwave multiplex radio transmission equipment is high and the cost of the microwave multiplex radio transmission equipment in the cost of the privately-owned radio communication system equipment is high. When handling the microwave multiplex radio transmission equipment, a high-degree operator qualification is required. It is hard for an independent user to introduce it.
The above-described problems (1) to (3) can be improved by connecting the control station and the relay station with the radio channel. When the control station and the relay station are connected by the radio channel, it takes time to process a signal for connection with the relay station by the radio channel, as described later. There arises a new problem that there is a difference in line connection facilitation between the mobile station in the communication zone of the control station and the mobile station in the relay station zone.